Chronic rejection represents the most important risk factor for poor long-term survival in clinical heart and lung transplantation [1, 2]. In most cases, the ultimate cause of graft loss is a slowly developing fibro-occlusive disease, whereby spaces within the organ fill up with collagen-forming cells and fibrous material which in turn blocks passage of blood and air, ultimately destroying the functional capacity of the graft. When identifiable, infiltrates during chronic rejection are relatively enriched for monocytes/macrophages, and tend to lack Thi cytokines [e.g. IL-2 and IFN-y] that predominate in the eariy acute rejection phase [3]. Unfortunately, the immunosuppressive [IS] drugs currently in use for transplant patients, mainly calcineurin inhibitors [CNI], corticosteroids and purine analogs, can reverse and prevent acute rejection but do little to halt the progress of fibro-obliteration. One possible reason for the ineffectiveness of CNI drugs in chronic rejection is that CNI drugs target the calcineurin/NFAT pathway that induces IFN-y and IL-2, inhibiting function of allospecific Th1 cells and 008 cytotoxic T lymphocytes. While these cleariy play a key role in acute rejection of allografts, IFN-y and IL-2 are also critically important for tolerance induction via apoptosis of T effector cells and tolerance maintenance by T regulatory cells [4][5]. Another possibility is that the critical T effector cell in chronic rejection may be Thi 7 rather than Thi cells; downstream targets of IL-17 receptor signaling are resistant to conventional IS drugs. For example, in studies of IL-17-driven production ofthe neutrophil chemokine IL-8 by bronchial smooth muscle cells, CNI drugs cyclosporine and tacrolimus, as well as corticosteroids and mycophenolate, were completely ineffective. Interestingly, the commonly used macrolide antibiotics erythromycin and azithromycin could effectively block this response, possibly accounting for their palliative effects on brochiolitis obliterans syndrome (BOS) in lung transplant recipients [6]. Evidence that Th17 cells can cause both acute and chronic rejection of organ transplants is mounting. Using mice knocked out for T-bet, a nuclear transcription factor essential for the Thi lineage [7], two groups have recently shown a pathogenic role for IL-17-producing T cells in the MHC class IIoniy mismatched bm12->B6 mouse heart transplant model, and in the fully allogeneic BALB/c-^B6 model. An indolent acute rejection response and slowly developing vasculopathy response to a Class 11 mutant bm12 heart Tx in the wt B6 recipient was mediated primarily by CD4/Th1 cells; absence of T-bet resulted in a rapid acute rejection response characterized by IL-17 producing T cells clustered around vessels and co-localized with abundant polymorphonuclear cells (PMNs) in the graft [8]. In grafts that survived the acute phase, chronic rejection developed with a more rapid onset in the absence of T-bet than in the wt B6 recipients. In the case ofthe fully allogeneic BALB/c heart transplant, Burrell et al. [9] reported a vigorous acute rejection in both wt and T-bet deficient hosts; however, Thi lymphocytes and macrophages were the predominant graft infiltrating cells in the wt B6 recipients, whereas in the T-bet knockout recipients, T cells producing IL-17 were co-localized with PMNs in the graft. Interestingly, the T effector cell in the fully allogeneic model was a CD8*, Tc-17 rather than a CD4* Th17 type cell. While these studies established that Th17 cells can indeed reject allografts, albeit by a different mode of graft injury (PMN-based, rather than CTL/macrophage-based), it nonetheless left open the question of whether Th17 cells mediate acute and chronic rejection of allografts in a normal host. Wilkes and colleagues were the first to show that autoimmunity to col(V) developed alongside alloreactivity in rat lung allotransplant [F344->WKY] recipients [10]. Subequently, they showed that lL-17-producing T cells present in lymph node cells from col(V)-pre-sensitized rats mediated acute rejection in rat lung isografts [WKY->WKY]; susceptibility ofthe isograft, and resistance ofthe native right lung to acute rejection correlated with expression ofthe target antigen in the extracellular matrix [11]. Two studies from the collaborative members of this program project team confirmed the importance of Th17 responses to col(V) in human lung transplantation. The first, a 7-year clinical study of 54 lung transplant patients, showed that cell-mediated immunity specific for col(V), characterized by dependence on IL-17 but not IFN-y, develops in lung transplant recipients prior to the onset of BOS [12]. Patients who failed to develop anti-col(V) responses in their peripheral blood mononuclear cells [PBMC] never developed severe BOS, and only 2/22 had even the mildest form [BOS-1]. So powerful was the association of Th17-dependent col(V)-specific autoimmunity with severe BOS onset that the Risk Ratio was 9.8 after a single timepoint of col(V) immunoreactivity in PBMC. For comparison, the BOS risk associated with HLA-DR mismatch and anti-HLA antibody formation was on the order of 1.5-2.0 [12]. The causative role of col(V)-specific Thi7 immunity in obliterative bronchiolitis (OB) was tested in the rat model by Dr. Wilkes' lab. Histology of the WKY lung isograft 30 d after transfer of col(V)-sensitized syngeneic lymph node cells were consistent with OB [12]. In the second clinical lung transplant study, Bobadilla et al. [13] found that 9/38 end-stage lung disease patients had a strong IL-17-dependent immune response to col(V) prior to transplant. A majority [6/9] were patients with idiopathic pulmonary fibrosis [IPF], suggesting possible etiology for this disease. Col(V) immunoreactivity was a highly significant risk factor for primary graft dysfunction [PGD] at 6 - 72 hours post-transplant. Only cold ischemia time, a well-known predictor of PGD, was as strong a risk factor in a multivariate analysis [13]. These data suggest that Thi7 cellular immunity to col(V), possibly augmented by anti-col(V) antibody [14], can mediate acute as well as chronic graft injury. The clinical monitoring studies to test the hypothesis of a causative link between col(V)-specific cellular immunity and BOS began at the UW-Madison in 1999. We chose a novel in vivo test that was introduced that same year: the trans-vivo delayed type hypersensitivity assay (TV-DTH) [15]. In retrospect, if we had chosen to monitor Th1 cells and cytokines IFN-y or IL-2 [standard at the time] we might never have found the human col(V)-specific immune response. But because we chose TV-DTH, we had a test that [unbeknownst to us then] could detect both Thi and Th17 cell-mediated immune responses. By 2007, we had come to appreciate that the neutralization of IL-17 and the removal of monocytes from PBMC, both of which had no effect on a recall TV-DTH response to tetanus toxoid [TT], completely abolished the response to col(V) [13][see also Preliminary Data]. That same year, two landmark studies of human Th17 cells were published [16,17]. In accordance with our findings regarding Thi 7 effector function in col(V)-specific TV-DTH, these articles noted that human Th17 development and propagation in vitro is strictly monocyte-dependent, and required IL-6 and IL-ip. IL-6 is the key cytokine driving Thi 7 inductionin both mice and humans, and graft donor-derived IL6 is an important local contributor to heart allograft rejection [18].